Influence of environmental changes in the tidal flats on the filtration and respiration of bivalve mollusks

Abstract Manila clam Ruditapes philippinarum and the other filter feeding bivalves in tidal flat are not only commercially important as seafood, but also ecologically significant because of their filtration activity. The volume of water filtration by bivalves in Ariake Sound is estimated to be equivalent to daily water exchange on the tidal flat in 1970s. However, the annual catch of Japanese littleneck has been decreased during the past 20 years in Japan. Especially, rapid decrease in the clam population in Ariake Sound since 1980s forced to depress the nationwide production. Recent coastal changes such as land reclamation, dike, port, barrage, and dam construction presumably brought about the environmental impact for filter feeding bivalves through water and sediment movement. Higher intertidal zone and supralittoral zone are intercepted by artificial structure such as dike and breakwater. Consequently suspended sediments are prevented from depositing at the higher intertidal zone and are drifted in littoral zone. High concentration of mud particles suppresses the water clearance of the clams. On the other hand, reduction of water current by barrages encourages the stratification. Hypoxia and anoxia often occur in subtidal zone of eutrophied sheltered coast under the stratified layer in summer. Complex effects of mud increase and oxygen shortage are considered to be physiologically harmful to filter feeding bivalves. The ecological function of tidal flat has been destroying and it disturbs the recovery of the bivalve resources.